Magnetic recording mediums are widely used in audio tapes, video tapes and floppy disks. It has been attempted in recent years to develop magnetic recording media which enable high density recording to be carried out and which have excellent electromagnetic characteristics capable of providing high image and high sound quality. Various attempts have been made to obtain excellent electromagnetic characteristics. With an increase in the use or application of the magnetic recording media and with an increase in the consumption thereof as media in large quantities, it has been demanded to produce the magnetic recording media at a lower manufacturing cost. Typical methods meeting these requirements include a method wherein loading is increased, a method wherein coercive force is increased, and a method wherein the surface of the medium is made smoother. Further, a multi-layer structure is extensively used to make a more effective recording in the direction of the depth of the magnetic layer. This is because the upper layer has the characteristics for high image quality, and the lower layer has the characteristics for high sound quality. In this approach, a suitable ferromagnetic powder can be used.
The advantage of the multi-layer structure is that the media can be inexpensively produced because a suitable material can be used depending on the respective layer. It is further required that the reproduced output be increased in order to achieve a high image and high sound quality, and that the modulation noise be decreased to improve S/N. This can be attained by improving the loading degree and the smoothness of the magnetic recording medium.
When a multi-layer structure is used, high output can be obtained over a wider frequency range by increasing the coercive force of the upper layer and lowering the coercive force of the lower layer, as described in JP-A-59-172144 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-61-214223, and JP-A-57-69528. The specifications of JP-A-58-56230, JP-A-58-56231, JP-A-58-70429, JP-A-1-106333 and JP-A-58-17539, disclose that noise is reduced by using fine ferromagnetic powder in the upper layer. Further, JP-A-58-53024 discloses that the upper layer is highly filled to obtain high output. Additionally, attempts have been made by paying attention to the size and shape of ferromagnetic powder used in the upper layer, as described in JP-A-57-98135, JP-A-63-187419, JP-A-64-19524, JP-A-2-192019 and JP-A-2-254626. The multi-layer structure is very effective in improving electromagnetic characteristics as mentioned above. However, with the development of high density recording, the degree of the improvement in the electromagnetic characteristics for magnetic recording mediums currently used is still insufficient.
For example, ferromagnetic powder having a finer particle size has been used to reduce noise. In order to reduce noise, the length of the major axis of ferromagnetic powder must not be longer than 0.3 .mu.m. Embodiments using ferromagnetic powder wherein the length of the major axis is not longer than 0.3 .mu.m are disclosed in JP-A-63-187419 and JP-A-64-19524. However, it is difficult to disperse and orient the ferromagnetic powder having such a fine particle size. Hence, it is very difficult to increase the loading degree of the powder. Accordingly, output is lowered and sufficient characteristics cannot be obtained, even though noise can be reduced.
Under such circumstances, the present inventors have eagerly made studies to solve the aforementioned problems found in the art, and have found that when the upper layer in the multi-layer magnetic recording mediums have a specific relationship between the thickness of the upper layer and the shape of retromagnetic powder, electromagnetic characteristics can be greatly improved. The present invention is based on this finding.